1. Field of the Invention
The invention relates to an image forming composition which comprises a) a ketal compound of the class of ketals known as 1,1-di[(alkylphenoxy)ethoxy]cyclohexanes, b) a near-infrared absorbing dye, and c) an alkali-soluble polymer. The ketal compound has hydrophilic groups and lipophilic groups. In general, the ratio of the molecular mass of the hydrophilic groups relative to the molecular mass of the lipophilic groups determines the solubility of the ketal in different solvents. The ketal compound is designed to be partially or totally water-soluble in an aqueous alkaline composition. Such image forming compositions are useful as part of positive-working, lithographic printing plates that are imagewise exposed to near-infrared radiation and then developed.
2. Description of the Related Art
There is great commercial interest in producing positive-working photographic elements, such as lithographic printing plates, having image forming composition coatings that are digitally imageable using near-infrared exposure. With positive-working plates, once the coating is imagewise exposed to such radiation, the exposed areas are easily removed, while the unexposed areas remain as the image. Positive-working, imageable compositions containing novolac or other phenolic polymeric binders and diazoquinone imaging components have been prevalent in the lithographic printing plate and photoresist industries for many years. Imageable compositions based on alkali-soluble polymers with various dissolution inhibitors and infrared radiation absorbing compounds are also well known. The dissolution inhibitors are believed to prevent dissolution by hydrogen bonding of the normally alkali-soluble polymers prior to imaging. During actinic exposure, it is generally believed that the mechanism of image formation occurs by an increase in alkaline solubility due to a disruption or breaking of the hydrogen bonds.
It is well known in the art that diazonaphthoquinone sulfonates are excellent dissolution inhibitors for novolacs, which are condensation products of phenols with formaldehyde. These novolacs generally are soluble in aqueous alkaline solutions. Imagewise exposure of a diazonaphthoquinone sulfonate in a coated novolac-containing layer produces an indene carboxylic acid, which does not inhibit the dissolution of the novolac, thereby creating a solubility difference between the exposed areas and the nonexposed areas. The exposed areas can be removed with an alkaline developer. This chemistry is described in much greater detail by R. Dammel in Diazonaphthoquine-based Resists, 1993, SPIE—The International Society for Optical Engineering (Bellingham, Wash.). Typical active absorption bands for the diazonaphthoquinone sulfonate lie between 350 nm and 400 nm. Therefore, these compounds and their coatings should be stored in the dark or at least under yellow light conditions, namely, with light which has been filtered to block light below about 500 nm.
It is also known in the art that dissolution inhibitors having acid-cleavable C—O—C groups can be used in combination with novolacs. Representative of such compounds are 2-tetrahydropyranyl ethers described in U.S. Pat. No. 3,779,778; ortho-carboxylic acid esters described in U.S. Pat. No. 4,101,323; polyacetals described in U.S. Pat. No. 4,247,611; and ketals described in U.S. Pat. No. 6,165,676. These compounds prevent dissolution of normally alkali-soluble phenolic polymers in alkaline developer solutions. Moreover, these dissolution inhibitors are normally mixed with photolytic acid-generating compounds in the image forming composition. Upon imagewise exposure of the composition to actinic radiation, an acid is released from the photolytic acid-generating compound that then catalyzes the decomposition of the dissolution inhibitors in the exposed regions. When this occurs, the exposed region can then be selectively dissolved in an aqueous-based, alkaline developer. Typical acid-generating compounds include trichloromethyl triazines whose absorption range extends to 500 nm. The typical absorption maximum is the range from about 330 nm to about 450 nm. Examples of such triazines are given in U.S. Pat. No. 4,619,998, and are sensitive to violet light (390-450 nm). Therefore, these compounds, like the diazonaphthoquinones, must be stored in the dark or at least under yellow light conditions. Because diazonaphthoquinones, trichloromethyl triazines, and other acid generators do not absorb in the near-infrared region, infrared radiation absorbing compounds are normally added to the image forming composition. Near-infrared absorbing dyes are well known. Examples of such dyes are given in U.S. Pat. No. 7,147,995.
U.S. Pat. No. 5,340,699 discloses an image forming material comprising a novolac binder, a resole polymer, infrared absorber, and an acid generator. The material is positive-working when an optional heating step after imagewise exposure and before development is not applied. The material is near-infrared sensitive, ultraviolet sensitive, and partially visible sensitive.
The dissolution inhibitors having acid-cleavable C—O—C groups are believed to prevent dissolution by hydrogen bonding of the normally alkali-soluble phenolic binder polymers prior to imaging. During actinic exposure, it is generally believed that the mechanism of image formation occurs by an increase in alkaline solubility due to the cleavage of the C—O—C groups of the ketal by the formed acid. Simple ketals of 1,1-dialkoxycyclohexane are generally poor dissolution inhibitors, due to the lack of hydrogen bonding sites. Examples of simple ketals include 1,1-dimethoxycyclohexane (Mw 144), 1,1-diethoxycyclohexane (Mw 172), and 1-1-di(1-methoxyethoxy)cyclohexane (Mw 200).
A particular group of ketal dissolution inhibitors of the class 1,1-di[(alkylphenoxy)ethoxy]cyclohexanes having the general formula [(R′)rC6H(5-r)O(CH2CH2O)m][(RR″)5C6H(5-s)O(CH2CH2O)n]C6H10 wherein m and n are independently an integer of from 1 to 5; r and s are independently an integer of from 1 to 3; and R′ and R″ are independently a hydrogen atom or an alkyl group having 1 to 5 carbon atoms are described in U.S. Pat. No. 6,165,676; U.S. Pat. No. 6,391,512; U.S. Pub. No. 2007/0172758; and U.S. Pub. No. 2009/0075201. The alkyl groups of R′ and R″ may be straight or branched, and include methyl, ethyl, propyl, butyl, and pentyl groups. The specific example of such ketals in the above-mentioned patents and patent applications is the ketal wherein m and n are 1; and R′ and R″ are hydrogen atoms. The formula for this specific ketal is [C6H5—O—(CH2CH2O)][C6H5O(CH2CH2O)]C6H10. This 1,1-di(2-phenoxyethoxy)cyclohexane (CAS 115815-82-2) is water insoluble. Strong acids will decompose this ketal into cyclohexanone and 2-phenoxyethanol, which has a low solubility of 27 g/l in water. It is desired that the compounds in the photosensitive layer after exposure to near-infrared light be soluble in an aqueous developer. Due to their water− insoluble characteristics, neither this specific ketal nor its acid-decomposed part is suitable for an aqueous developer. Furthermore, due to the relatively low molecular weight of 356 for this specific ketal, lithographic coatings with this ketal tend to have lower printing durability in the image areas of the printing plate. In addition, due to the lack of lipophilic alkyl groups on this specific ketal, coatings with this ketal tend to not accept printing inks well. The preferred acid-generating compounds for these ketals in the examples of the four above-mentioned references are trichloromethyl triazines, which are ultraviolet sensitive and partially visible sensitive. It is desired that the image forming composition be sensitive to near-infrared radiation but not to violet radiation, namely, do not require an acid generator. Therefore, a special group of ketals that are partially or totally water-soluble have been developed so that the addition of acid-generating compounds is not required.